- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7250) Unit Title: PLANT REPRODUCTION ____________________________________________________________________________ (CLF7251) Topic: SEXUAL REPRODUCTION Time Taught in Year(s) 5 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (D-1-5) - Explain the difference between sexual and vegetative propagation. (D-5) - Demonstrate an application of both sexual and vegetative propagation. (D-4,5) - Discuss how sexual and asexual propagation are used in agriculture. (D-1) - Define pollination. (D-1) - Describe two situations of pollen transfer that can be called "self-pollination". (D-2) - Discuss four methods by which pollen is distributed. (D-4) - List and describe in order of occurrence, the major events that take place in sexual reproduction of flowering plants, including pollination, fertilization and seed production. (D-5) - Describe the advantages and disadvantages of seed (sexual) propagation as compared to vegetative propagation. Special Materials and Equipment: Cutting material for rooting, and roots for root cuttings, rooting media and containers. References: Hartman, H., Flocker, W., and Kofranek, A. PLANT SCIENCE, GROWTH DEVELOPMENT AND UTILIZATION OF CULTIVATED PLANTS. Evaluation: Quiz by instructor TOPIC PRESENTATION: SEXUAL REPRODUCTION A. Plant propagation is the multiplication of plants by both sexual and asexual means. 1. The goal of propagation is to increase the number of plants with valuable combinations of traits. 2. Sexual propagation is by seed. 3. Asexual propagation is the increase of plants by vegetative methods. (Cuttings, budding and grafting, layering, separation, and division.) B. Most plants are started from seed. 1. Seed propagation is normal for vegetables, flowers, grains and forest trees. Generally, sexual propagation is the method of choice because of its several advantages: a. Propagation by seed is more economical, b. propagation by seeds is easier, c. most seeds can easily be stored for long periods of time, and d. seeds are less likely to transmit diseases to their offspring than are plants started by vegetative means. 3. There are disadvantages of using seeds. a. In some species, seeds are variable or inconsistent (don't breed "true") in genetic makeup, (e.g., potato); b. some plants are slow to mature when grown from seed; and c. some plants do not produce viable seeds. (e.g., Hydrangea, and navel orange, banana.) 4. Seedlings of different species are used as rootstocks for many tree species. a. They are used because they have one or more of the following characteristics. 1) dwarfing, 2) tolerance to drainage problems, 3) resistance to disease, and 4) compatible with the selected variety. 5. Seedlings are also used in plant breeding, to make plants with new combinations of traits. a. As parents, the breeder selects two plants that have different desirable traits; for instance, one parent might be disease resistant while the other produces especially good fruits. b. A cross between the parents leads to plants with various combinations of traits. The breeder selects those individuals that have all the desired traits (for instance, both disease resistance and high fruit quality). C. Seeds are produced in flowers. 1. The flower consists of both sexual and sterile parts. 2. Sterile parts include the receptacle, to which other parts are attached; and the sepals and petals, which protect the flower bud and attract pollinating animals. 3. Fertile parts are the male stamens, which make pollen, and the female pistils, which contain egg-bearing ovules. D. The seed is the result of complex growth and developmental events. 1. Pollen formation is a first step in this development. a. Pollen is made in anthers, which are part of the flower's stamens. b. A pollen grain consists of two or more cells in a protective wall. c. Pollen grains can travel through dry air, carried by animals or wind, without being killed by dehydration. d. The pollen grain plays a part similar to the animal's sperm cells, travelling to the female structures where eggs occur. e. When a pollen grain meets a pistil, it grows a pollen tube that carries male nuclei into the pistil. f. A single anther can produce thousands or millions of pollen grains. g. Pollen grains of various plant taxa differ in size, shape, color, and surface ornamentation. Thus, they can be used in classifying plants. 2. Another first step is the production of pistils with ovules. a. A pistil has a sticky tip (the stigma) that captures pollen, a long tube (the style) that guides the pollen tube, and an enlarged basal ovary that contains ovules. b. An ovule consists of an egg and several other cells that aid in reproduction, enclosed in layers of protective cells called integuments. c. A single pistil may contain many ovules. 3. Pollination is the next step in the development of seeds. a. Pollination is the transfer of pollen from the anther to the stigma (top part of the pistil) of a flower. b. Once the pollen grain has reached the stigma it stays in place because of a sticky exudate produced by the stigma. c. This fluid also provides nourishment for the pollen grain. d. The pollen grain sends out a long tube (pollen tube) that grows down the style of the flower and finally reaches the ovule within the ovary. 4. The next step is fertilization. a. A nucleus from the pollen tube unites with the egg nucleus in an ovule, making a zygote (the first cell of the new plant). 1) The zygote gets half its genes from the plant that made the pollen, and half from the parent that made the egg. b. A second pollen nucleus combines with nuclei of the ovule (not the egg or zygote) to make a cell that will form nutritive tissue called endosperm. c. Such double fertilization only occurs in flowering plants. 5. Each fertilized ovule develops into a seed. a. The zygote grows into an embryo by cell expansion, mitotic division, and cell differentiation. b. The other fertilized cell develops into nutritive endosperm tissue by expanding, accumulating foods, and dividing repeatedly. c. The integuments develop into a seed coat that hardens and may accumulate waterproofing compounds. d. In some seeds, the embryo consumes the endosperm before the seed is mature. 1) This is true of legumes (beans and peas), squash, and many other plants. 2) The endosperm cells break down, and the embryo's cotyledons absorb the released food. 3) The cotyledons thicken and retain the food until the seed germinates. e. In other seeds, the endosperm persists until the seed germinates. 1) This is true of cereals such as corn. 2) At germination, the endosperm breaks down, and the released food is absorbed by cotyledons. 6. Agricultural practices may affect the number and quality of seeds produced. a. Each fertilized ovule produces one seed, so the number of seeds depends on the effectiveness of pollination. b. This can affect fruit quality in multiple-seeded fruits such as ears of corn. c. Growers may increase the yield in some crops by placing beehives in the fields prior to bloom (e.g., alfalfa, cucumbers, squash, almonds). d. Seed quality may depend on whether the ovules are self- fertilized or cross-fertilized. 1) In self-fertilization, pollen and ovules are made by the same plant, or by plants of the same clone. 2) In cross-fertilization, pollen and ovules come from genetically different plants. 3) Repeated self-fertilization results in inbreeding. Inbred plants often show reduced vigor and make fewer fruits and seeds. 4) Cross-fertilization often leads to vigorous, highly productive plants, an effect called hybrid vigor. 5) Cross-pollination is useful in plant breeding, and can be controlled by removing anthers from a plant before pollination, so that the grower can supply pollen from another selected plant. 6) Some plants cannot self-pollinate. a) Some plants have distinct male and female individuals. b) Some varieties have barriers that prevent pollen tube growth if the pollen comes from a plant with the same genes as the ovule-making parent. This self-incompatibility occurs in some fruit and nut trees. c) In self-incompatible crops, the grower must plant two compatible varieties together: a pollinating variety and a seed-producing variety. __________________________________________________________ ACTIVITY: 1. Collect pollen grains from several different plants. View them in a microscope noting different sizes, shapes and colors. 2. Germinate pollen grains in a sugar solution. Observe growth of pollen tubes. Determine germination percents. 3. Dissect flowers of local crop or ornamental plants. Separate flower parts (petals, stamen, pistil, and sepals). Observe their characteristics of shape, sizes, and colors. __________________________________________________________ 7/20/90 sg #%&C